As gas turbine engine technology advances and engines are required to be more efficient, gas temperatures within the engines continue to rise. However, the ability to operate at these increasing temperatures is limited by the ability of the superalloy turbine blades and vanes to maintain their mechanical strength when exposed to the heat, oxidation, and corrosive effects of the impinging gas. One approach to this problem has been to apply a protective thermal barrier coating which insulates the blades and vanes and inhibits oxidation and hot gas corrosion.
Typically, thermal barrier coatings are applied to a superalloy substrate and include a bond coat and a ceramic top layer. The ceramic top layer is applied either by the process of plasma spraying or by the process of electron beam physical vapor deposition (EB-PVD). Use of the EB-PVD process results in the outer ceramic layer having a columnar grained microstructure. Gaps between the individual columns allow the columnar grains to expand and contract without developing stresses that could cause spalling. Strangman, U.S. Pat. Nos. 4,321,311, 4,401,697, and 4,405,659 disclose thermal barrier coatings for superalloy substrates that contain a MCrAlY layer, an alumina layer, and an outer columnar grained ceramic layer. Duderstadt, et al., U.S. Pat. No. 5,238,752, and Strangman copending U.S. patent application Ser. No. 06/603,811, U.S. Pat. No. 5,514,482, disclose a thermal barrier coating for a superalloy substrate that contains an aluminide layer, an alumina layer, and an outer columnar grained ceramic layer.
A disadvantage to ceramic top layers applied by commercially available EB-PVD processes is that their thermal conductivity is about two times higher than the thermal conductivity of ceramic top layers applied by the plasma spray process. High thermal conductivity is undesirable and is believed to result from the deposition of relatively high density columnar grains with little internal microporosity.
Accordingly, there is a need for a thermal barrier coating and method therefor to be applied by EB-PVD that has a lower thermal conductivity.